Vasculitis is a pathophysiologic process which is involved in a spectrum of clinical syndromes. The underlying mechanism of this process is in most cases an aberrant immunologic response. The LIR is currently studying prospectively the largest group of patients with the vasculitic syndromes in the world. On the basis of clinical, pathophysiologic, immunopathogenic and therapeutic results obtained over the past 18 years, we have designed a revised categorization scheme for the vasculitides which has now reached worldwide acceptance. In addition, we have described a new vasculitis syndrome which we have termed the polyangiitis overlap syndrome. We have developed and instituted aggressive chemotherapeutic regimens consisting of chronically administered cyclophosphamide together with alternate-day corticosteroids in several, formerly universally fatal diseases such as Wegener's granulomatosis. In this regard, we are now following over 140 patients with Wegener's granulomatosis in which we demonstrated a 93% remission and cure rate. We have now applied these approaches with remarkable success to other of the vasculitic syndromes such as systemic vasculitis of the polyarteritis nodosa group, isolated central nervous system vasculitis, Takayasu's arteritis, the acute vasculitis of Sjogren's syndrome, and lymphomatoid granulomatosis. The patient populations studied in the vasculitis protocol have been utilized to precisely delineate aberrancies of lymphocyte activation and immunoregulation seen in these diseases. In addition, the precise effects of various therapeutic regimens, particularly corticosteroids and cytotoxic agents, on human lymphoid cells have been described. In this regard, we demonstrated the exquisite and selective sensitivity of certain phases of the B cell cycle to cyclophosphamide therapy, an observation which might help explain its efficacy in certain diseases characterized by hyperreactivity of B cell function. Furthermore we have delineated the spectrum of effects of corticosteroids on the various phases of the human B cell cycle.